1. Field of the Invention
The invention relates to detection of minute amounts of trace elements in a field environment, using laser induced spectroscopy. In particular, the invention relates to a mobile apparatus which allows in situ measurement and data collection of ground water trace element pollutants.
2. Related Art
Raymond Measures in Laser Remote Chemical Analysis discloses laser remote sensing including scattering, absorption and fluorescence techniques. Flame spectroscopy has been used to identify metal substances in an analysis sample. A sample is heated to temperatures in the order of several thousands of degrees, causing the metals to emit characteristic resonance lines, which can be used with spectroscopy techniques to detect the presence of very low concentrations of such metals.
In September 1989, the United States Environmental Protection Agency, Office of Water Regulations and Standards, Industrial Technology Division (Office of Water) issued draft Method 1620: Metals by Inductively Coupled Plasma Atomic Emission Spectroscopy and Atomic Absorption Spectroscopy, hereinafter EPA Method 1620. Inductively coupled plasma atomic emission spectroscopy (ICP) is among the methods which have been employed to detect trace elements, such as metals, in solutions, such as ground water and soil samples. ICP has been used for both for simultaneous and sequential determination of trace elements in solution. In the ICP method, samples are nebulized by an ultrasonic instrument, creating an aerosol. The aerosol is transported to a plasma torch, where excitation occurs. Characteristic atomic line spectra are produced by a radio frequency (RF) coupled plasma. The spectra are dispersed by a grating spectrometer and the intensities monitored by photomultiplier tubes. The tube photocurrents are processed and controlled by a computer system.
In addition to the need for transporting the sample, it must also be filtered, acidified and filtered for analysis. Dissolved elements, those which will pass through a 0.45 m membrane filter, are determined in samples that have been filtered and acidified. Background correction techniques may be needed and appropriate steps must be taken in all analyses to assure that interferences are taken into account.
EPA Method 1620 also documents Cold Vapor Atomic Absorption (CVAA) techniques employed for the analysis of mercury. This flameless procedure is based on the absorption of radiation at 253.7 nm by mercury vapor. Using this method, mercury compounds are oxidized and the mercury is reduced to the elemental state and aerated from solution in a closed system. The mercury vapor passes through a cell positioned in the light path of an atomic absorption spectrophotometer. Absorbance (peak height) is measured as a function of mercury concentration. Organic mercurials which may be present will not respond to the CVAA technique, unless they are first broken down and converted to mercuric ions. This requires an oxidation step, using potassium persulfate as the oxidant. In addition, a heating step is required for methyl mercuric chloride when present in or spiked into a natural system. Thus, this method also requires a complicated sample preparation.
EPA Method 1620 also documents GFAA Spectroscopy for analysis of water and soil/sediment samples, as a method for multi-element determination of trace elements in solution. Using this technique, a few microliters of the sample are evaporated at low temperature to remove the solvent from the sample and then ashed at higher temperature (2000.degree.-3000.degree. C.), for example, by a heated conductor. Atomization occurs in a few milliseconds to a few seconds. The absorption or fluorescence of the atomized particles can then be measured in the region above the heated conductor. At the wavelength at which absorption (or fluorescence) occurs, the detector output rises to a maximum after a few seconds of ignition. This is followed by a rapid decay back to zero as the atomization products escape into the surroundings. A high speed recorder is used to monitor the change.
Many organic contaminants in a ground water analysis sample have an aromatic (ring) molecular structure, which efficiently absorbs light and later emits absorbed ultraviolet energy as fluorescence. The intensity, wavelength and time characteristics of the fluorescence waveforms can be used to identify types and concentrations of organics in an analysis sample.
These conventional techniques for the analysis of contaminants in a ground water or other analysis sample involve the application of routine repetitive analysis, such as gas chromatography and mass spectroscopy on samples taken from wells. Such monitoring requires the use of sampling procedures, which must avoid introducing further contaminants into the sample. These conventional techniques are labor intensive because of the long time required for sample preparation, analysis and storage. These approaches are also constrained by the need to remove a sample from its environment and heat the sample to a high temperature. As a result, a labor intensive sampling and analysis process is also required to detect the presence of metals in analysis samples.
Laser produced plasmas, plasmas produced by high power lasers, have been used in various scientific and industrial applications, in particular, material manufacturing, coating materials, painting and in drilling. However, laser produced plasma spectroscopy has not heretofore been used in the detection of pollutants or contaminants in analysis samples.